The yarns for footwear upper or industrial uses, as it is well known, are mostly polyester, nylon, acryl resin, or the like. The fabrics processed from those yarns are not only poor in durability and wear resistance, but also problematic in many aspects including adhesiveness, so they are unfit to use for high level functionality such as of footwear fabrics or the like.
Currently, a solution to this problem is a yarn (hereinafter, referred to as “coated yarn”) of which the surface is coated with a thermoplastic resin in order to enhance the strength of the yarn. Such a coated yarn is generally fabricated by applying a coating of a thermoplastic resin such as PVP or PP or a thermoplastic polyurethane resin on a yarn like polyester or nylon in a dice using a general extruder.
But, when using a general thermoplastic resin, it is difficult to control the amount of the coating, particularly making it impossible to use a small amount of the coating, ending up producing a thick coated yarn having a high denier count above 350 and leading to deterioration in durability and wear resistance.
In an effort to solve this problem, as can be seen from the following patent documents 1 to 4, the inventors of the present invention have been making sustained research and development on a fabrication method for coated yarn, a compound for coated yarn, a thermoplastic polyurethane coated yarn, etc. since 2012.
These prior patents may produce coated yarns with excellences in wear resistance, adhesiveness, water resistance, molding properties, etc. Yet, the coated yarns of the prior patents are bound to have a core like polyester or nylon, which results in large thickness, making it impossible to realize a coated yarn of thinness.
Besides, the TPU coated yarns disclosed in the prior patents have such a viscosity not as high as polyester or nylon due to the characteristics of the thermoplastic polyurethane, so a thickening agent is necessarily used for the sake of smooth implementation of the extrusion process.
For single yarns, however, things are different from the TPU coated yarns of the prior patents. Silica of normal size may be used as a thickening agent for thick mono-filament yarns, but it cannot be used for thin multi-filament yarns, more specifically for multi-filament yarns having a denier count less than about 50. Even in the case of mono-filament yarns (for example, mono-filament yarns having a denier count of 50 to 350) using general silica, thread breakage (namely, the situation that the thread breaks or snaps) occurs inevitably in the drawing process. This makes it impossible to produce TPU yarns continuously and hence leads to low productivity.
Accordingly, silica of normal size as suggested above is unavailable in the continuous production of TPU yarns to draw mono- or multi-filament yarns comprised of thermoplastic polyurethane continuously. In particular, general silica is unfit to use as a thickening agent in drawing thin TPU yarns (namely, a single filament yarn that is less than 50 denier for multi-filament yarn and between 50 to 350 denier for mono-filament yarn) continuously without thread breakage.